A known mass flow controller is provided with a main passage and a flow measuring passage branched from the main passage. A pair of sensors are provided in the flow measuring passage, so as to measure a mass flow of fluid between the sensors. Unless a ratio of the mass flow in the flow measuring passage to that in the main passage is always constant, there is generated an error in measurement of the mass flow in the mass flow controller as a whole. Therefore, it is necessary to maintain laminar flow in the flow measuring passage and the main passage, so as to maintain the above ratio constant.
Such a mass flow controller is disclosed in Japanese Patent Laid-open Publication No. 60-56219, for example. Referring to FIG. 7 which shows such a prior art mass flow controller 70, the mass flow controller 70 is provided with a base 76 having a tapering circular passage in which a tapering plug 75 is accommodated with a tapering annular passage defined between the same and an inner circumferential surface of the tapering circular passage. Further, an inlet 73 and an outlet 74 of a flow measuring passage 72 open into the tapering annular passage. The plug 75 is not fixed in position causing nonuniformity in the cross section of the tapering annular passage. As a result, fluid flow into the flow measuring passage 72 becomes unstable.
Furthermore, a cross-sectional area of the tapering annular passage at the position of the outlet 74 is smaller than that at the position of the inlet 73. Accordingly, the flow in the tapering annular passage is contracted to cause the formation of very small vortex in the tapering annular passage. This vortex causes instability of the fluid flow into the flow measuring passage 72. Such a tendency of instability of the fluid flow becomes greater as the ratio of the fluid flow in the tapering annular passage to that in the flow measuring passage 72 becomes larger.
Further, it is difficult to form the tapering circular passage through the base 76.
It is accordingly a primary object of the present invention to provide a flow dividing structure of a mass flow controller which can maintain the fluid flow into the flow measuring passage constant and thereby ensure accurate flow measurement.
It is another object of the present invention to provide a flow dividing structure of a mass flow controller which can form a uniform cross section of an annular passage at the position of the inlet of the flow measuring passage to thereby further improve the accuracy of measurement of the fluid flow.
It is a further object of the present invention to provide a flow dividing structure of a mass flow controller which eliminates the need for taper machining of the base to thereby make manufacturing easy.
According to the present invention, there is provided a flow dividing structure of a mass flow controller, comprising a base provided with a main passage having an upstream circular passage, a downstream circular passage, and a tapering circular passage connecting said upstream circular passage with said downstream circular passage; a plug inserted in said main passage and provided with a first cylindrical portion for forming a first annular passage between the same and an inner circumferential surface of said upstream circular passage, a second cylindrical portion for forming a second annular passage between the same and an inner circumferential surface of said downstream circular passage, and a tapering frustoconical portion for forming a tapering annular passage between the same and an inner circumferential surface of said tapering circular passage; and a flow measuring passage connecting said upstream circular passage with said downstream circular passage so as to bypass said tapering circular passage.
With this construction, the flow measuring passage branched from the main passage is formed to connect the first annular passage with the second annular passage, both being parallel to an axis of the main passage, in such a manner as to bypass the tapering annular passage. Accordingly, a laminar flow can be formed in the first annular passage and the second annular passage, thereby stabilizing the fluid flow into the flow measuring passage and ensuring accurate flow measurement.
In a preferred form of the present invention, the first cylindrical portion and/or the second cylindrical portion of the plug may be formed with a plurality of radial projections extending in an axial direction of the plug, and the radial projections are in elastic contact with the inner circumferential surface of the upstream circular passage and/or the downstream circular passage. Accordingly, the cross section of the first annular passage and/or the second annular passage can be made uniform to thereby further improve the accuracy of measurement of the fluid flow.
In another preferred form of the present invention, a sleeve is inserted into a cylindrical bore formed in the base, so that an inner circumferential surface of the sleeve provides the inner circumferential surfaces of the upstream and tapering circular passages, and may additionally provide the inner circumferential surface of the downstream circular passage. Accordingly, it is not necessary to form a tapering bore in the base, thereby making manufacturing easy.
Other objects and features of the invention will be more fully understood from the following detailed description and appended claims when taken with the accompanying drawings.